In an example conventional Long Term Evolution (LTE) radio frequency (RF) transmitter, an oversampling digital-to-analog converter (DAC) block may be employed to convert a modem's digital baseband signals to analog. The DAC's output contains thermal and quantization noise to be filtered using a low-pass baseband filter. Also the DAC output contains alias of the baseband signal at the DAC's sampling frequency. This DAC alias may fall into an RX band in some carrier aggregation modes resulting in desensitization of the receiver path. Hence it would be beneficial to use a sharp attenuation in a baseband filter (BBF) to suppress DAC alias.
A notch filter may be employed to achieve this rejection at DAC sampling frequency. For instance, a passive resistive capacitive (RC) Twin-T notch is an available architecture which may be placed in series with a baseband filter to remove the DAC alias.
The output from the baseband filters may include in-phase (I) and quadrature (Q) phases of the analog signal. Some conventional systems use a harmonic reject mixer to receive the I-Q phases and to upconvert the signal. When a harmonic reject mixer is being used, it may employ multiple phases from the baseband filter apart from the usual I-Q phases. These extra phases may be generated by linearly combining I-Q phases through use of a passive resistive capacitive (RC) circuit. In an example using a harmonic reject mixer and a notch filter, the notch filter may be placed before the harmonic reject mixer and integrated with the baseband filter.
It would be desirable to save semiconductor die space by reducing the total number of components in the harmonic reject mixer and notch filter.